This invention relates to a method of growing a single crystal of .beta.-barium borate, .beta.-BaB.sub.2 O.sub.4, by a Czochralski technique.
It is known to grow a high purity .beta.-BaB.sub.2 O.sub.4 single crystal by the Czochralski method from a BaB.sub.2 O.sub.4 melt. The starting material for the melt is either a powder of .beta. phase BaB.sub.2 O.sub.4 (Journal of Crystal Growth, Vol. 106 (1990), 728-731) or sintered grains of .beta. phase BaB.sub.2 O.sub.4 powder (Journal of Crystal Growth, Vol. 114(1991), 676-682).
However, practical applications of the known growth methods encounter several problems. When the starting material of the melt is a .beta. phase powder, which may be sintered, it is convenient to use a platinum rod instead of a seed crystal. At the contact of the platinum rod with the melt a crystalline material nucleates on the platinum rod, but the nucleated material is a polycrystal of .beta.-BaB.sub.2 O.sub.4. To cause transformation of the nucleated polycrystal to a single crystal there is the need of first adjusting the diameter of the polycrystal to about 1.5 mm and then growing the crystal to a length of 5 to 10 mm while keeping the diameter of about 1.5 mm. This operation takes a considerably long time and requires intricate techniques, and often the operation fails by breaking of the growing crystal from the melt or by accidental transition of the .beta. phase to the .alpha. phase. Besides, the yield of acceptable .beta.-BaB.sub.2 O.sub.4 single crystals is relatively low because in many cases grain boundaries exist in the grown .beta.-BaB.sub.2 O.sub.4 single crystals.